A common problem when connecting networks is that of ensuring messages from one network are correctly routed to a destination on an other network. A particular example of this is in the connection of networks using Internet Protocol (IP) to an existing telephone switched circuit network (PSTN). The infrastructure uses very different technologies and protocol: for example a PSTN uses the SS7 protocol, and it is desirable to provide PSTN signalling over an IP network reliably.
A number of approaches to providing a reliable connection between IP and SS7 networks have been proposed by the Internet Engineering Task Force (IETF). These include SUA which defines the protocol for the transport of any SCCP user signalling over IP. Two main entities are defined: the signalling gateway (“SG”), which is the connection point between the SS7 and IP networks, and the application server (AS) which is the software application provided on the IP network which it is desired to make available over the SS7 network. The protocol may be used to connect SS7-based signalling end points (SEP) with an IP based AS thus allowing SS7 networks to access IP based applications.
A function performed by the signalling gateway is to receive SS7 signalling messages and direct them to the appropriate AS. To do so, the SG identifies a routing key (RK) defined by the SUA protocol, which uniquely identifies the AS in accordance with parameters in the SS7 message. In general, the RK parameters are found in the header of the SS7 message, although it may be desirable to take into account other elements of the message, such as some aspect of the message content, in order to direct the message to the appropriate AS.
Accordingly, each application server behaves like an SS7 SEP, including SS7 characteristic information including a point code and a subsystem number. When it receives a message and generates a response, it will include its point code and subsystem number in the response to permit the SS7 SCP point or signalling end point to know the origin of the response. In a signalling gateway, the SCCP layer defines services which are identified with an SSN, and so the SSN and PC corresponding to an application server may be used to appropriately route messages from the SS7 and IP networks.
A problem with such a system however, is that each application server must be configured with information which is required by the SS7 network. Further, one internet network service could be provided by a number of application servers and equally, an application server could provide many separate services, in which case the specific one-to-one matching of point codes and signalling entities required by SS7 signalling is inappropriate. If it is desired to add new application servers or services on an IP network, the signalling gateway and the new application servers must be configured accordingly to avoid instability or traffic routing errors.